On Wed, Apr 10, 2019 at 09:26:22AM +1200, Duncan Kinnear wrote: > ----- On 5 Apr, 2019, at 4:14 AM, Jeff Janes <jeff.janes@xxxxxxxxx> wrote: > > > On Thu, Apr 4, 2019 at 3:42 AM Duncan Kinnear <duncan.kinnear@xxxxxxxxxxxxxx | duncan.kinnear@xxxxxxxxxxxxxx ] > wrote: > > >> the EXPLAIN (ANALYSE, TIMING TRUE) of this query gives: > > >> Update on next_id (cost=0.14..8.16 rows=1 width=36) (actual time=0.057..0.057 > >> rows=0 loops=1) > >> -> Index Scan using next_id_pk on next_id (cost=0.14..8.16 rows=1 width=36) > >> (actual time=0.039..0.040 rows=1 loops=1) > >> Index Cond: ((id)::text = 'Session'::text) > >> Planning Time: 0.083 ms > >> Execution Time: 0.089 ms > > >> which is significantly less than 50ms. > > > The EXPLAIN ANALYZE doesn't include the time needed to fsync the transaction > > logs. It measures only the update itself, not the implicit commit at the end. > > DBeaver is seeing the fsync-inclusive time. 50ms is pretty long, but some file > > systems and OSes seem to be pretty inefficient at this and take several disk > > revolutions to get the data down. > > >> Now, if I point DBeaver to a VM server on the same gigabit network switch, > >> running version: > >> 9.5.3 on i386-pc-solaris2.11, compiled by cc: Sun C 5.10 SunOS_i386 Patch > >> 142363-07 2010/12/09, 64-bit > >> then the same query executes in about 2-3ms > > > That machine probably has hardware to do a fast fsync, has fsync turned off, or > > is lying about the safety of its data. > > Just a quick update. I tried performing a sequence of BEGIN; UPDATE ...; COMMIT; and I got the following log entries: > Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.640 NZST [29887] LOG: duration: 0.003 ms bind <unnamed>: commit > Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.690 NZST [29887] LOG: duration: 50.237 ms execute <unnamed>: commit > > So this confirms that the overhead is indeed happening in the COMMIT part. But how do I get more detailed logging to see what it is doing? commit is causing the fsync() Jeff mentioned. You could test that's the issue by comparing with fsync=off (please read what that means and don't run your production cluster like that). https://www.postgresql.org/docs/current/runtime-config-wal.html#GUC-FSYNC You could also put your XLOG on a separate FS (as a test or otherwise). Justin
